Well the Washington Post decided to find out:
It was 7:51 a.m. on Friday, January 12, the middle of the morning rush hour. In the next 43 minutes, as the violinist performed six classical pieces, 1,097 people passed by. Almost all of them were on the way to work, which meant, for almost all of them, a government job.
Each passerby had a quick choice to make, one familiar to commuters in any urban area where the occasional street performer is part of the cityscape: Do you stop and listen? Do you hurry past with a blend of guilt and irritation, aware of your cupidity but annoyed by the unbidden demand on your time and your wallet? Do you throw in a buck, just to be polite? Does your decision change if he's really bad? What if he's really good? Do you have time for beauty? Shouldn't you? What's the moral mathematics of the moment?
Many thanks to the lovely Susan for the tip.